This invention relates to a pump device, and, more particularly, it is concerned with a vane type vacuum pump device.
As a kind of conventional vacuum pump, there has been known one as disclosed in Japanese Unexamined Pat. publication No. 24307/1979 (which is shown in FIGS. 1 and 2 of the accompanying drawing). In the illustration, a reference numeral 1 designates a housing having an air intake port 1a, an air discharge port 1b, a cylindrically formed inner peripheral surface 1c, and a side wall 1d; a numeral 2 refers to a drive shaft rotatably supported in the housing 1. A numeral 3 denotes a rotor fixed on the drive shaft 2, having an outer peripheral surface 3a, and which rotates eccentrically within the housing 1 along with rotation of the drive shaft 2; 3b refers to a plurality of vane grooves formed in the rotor 3; a numeral 4 denotes a plurality of vanes, each having a side surface 4a inserted in each of the vane grooves 3b, slide-moving in the vane groove 3b in the radial direction of the rotor 3, and being in slide-contact with the inner peripheral surface 1c of the housing 1 with rotation of the rotor, to thereby forward under pressure a gas from the air intake port 1a to the air discharge port 1b. A reference numeral 5 designates a bearing to rotatably support the drive shaft 2; a numeral 6 refers to a bracket which supports thereon the bearing 5, and the side wall 6a of which is fixedly secured on the housing 1; reference numerals 7, 8 and 9 designate operating chambers defined by the outer peripheral surface 3a of the rotor 3, the inner peripheral surface 1c and the side surface 1d of the housing 1, the side wall 6a of the bracket 6, and a plurality of the vanes 4; a numeral 10 refers to an oil feeding port formed in one part of the housing 1, through which lubricating oil sent from an oil pump (not shown in the drawing) is fed into the housing 1. A reference numeral 11 represents an oil seal to prevent leakage of oil from the housing 1, and a numeral 12 refers to a clearance groove cut out in one part of the inner peripheral surface 1c of the housing 1.
In the conventional vacuum pump device of the above-described construction, when the air intake port 1a is rendered a high vacuum state by the operation of the pump, the operating chamber 8 is also rendered at a high vacuum condition. In this state, when the operating chamber 8 is opened to the air discharge port 1b which is at the atmospheric pressure, there takes place back-flow of air containing lubricating oil from the air discharge port 1b to the operating chamber 8 to cause cavitation destruction, cavitation noises, and vibration noises due to pressure waves. With a view to solving such problem, the conventional vacuum pump device has a clearance groove 12 cut in the inner peripheral wall of the housing 1. In more detail, when the vane 4 is passing by the clearance groove 12, air is sent into the operating chamber 8 in the high vacuum condition from the air discharge port 1b at the atmospheric pressure through the operating chamber 9 and the clearance groove 12 to bring the pressure in the operating chamber 8 closer to the atmospheric pressure until the operating chamber 8 becomes open to the air discharge chamber 1b, thereby preventing the above-mentioned cavitation destruction, etc. from taking place.
On the other hand, however, since the air is supplied into the operating chamber 8 from the air discharge port 1b at the time of passing-by of the vane 4 at the clearance groove 12, the operating chamber 8 is brought closer to the atmospheric pressure in a low speed rotational region, whereby a difference in pressure between the operating chamber 8 and the air discharge port 1b becomes small. However, since the operating chamber 7 is in the high vacuum condition, a large pressure is applied to the vane 4 separating the operating chamber 7 and the operating chamber 8 to cause one-sided or partial wear of the side surface 4a of the vane 4 due to slide-contact with the vane groove 36. Further, in a high speed rotational region, since the vane 4 very quickly passes by the clearance groove 12, a sufficient amount of air cannot be fed into the operating chamber 8 from the air discharge port 1b with the consequence that the cavitation destruction, and other undesirable phenomena cannot sufficiently be prevented.